It was the briefest of remarks in a wide-ranging discussion and concerned something he has no power to do, but the sheer unlikelihood of Adair Turner floating the Tobin tax made for the most eye-catching line in his contribution to yesterday's Prospect magazine. Lord Turner, after all, is not only chair of the Financial Services Authority, but a former investment banker and the very embodiment of the great and the good, with the looks and the accent to match. The Tobin tax, meanwhile, has been the rallying cry of long-hairs and leftists who are often hazy about what the City gets up to, but always have a strong hunch that it is up to no good.

The remarkable thing about Lord Turner's full comments is that they suggest he may be starting to feel the same way. The Tobin tax – a small percentage tariff charged on financial transactions – was originally proposed by the great American Keynesian, James Tobin, as a means of discouraging footloose flows of finance from playing havoc with exchange rates, though its latter-day advocates have talked it up as a nice little earner with which to tackle global injustice. Lord Turner mentioned it, however, in connection with a third objective – cutting the financial sector down to size. Despite his official remit to make London more competitive in world markets, he seems to have concluded that it has become bloated – taking too many bright people away from the rest of the economy, assigning many of them worthless tasks and paying them too much money. Along with stiffer capital requirements and other charges on banking profits, Tobin's tariff is thrown in as one further way to siphon some of the money to more worthwhile activities.

Some of this may be a response to the criticism that the FSA – a body the Conservatives are pledged to abolish – has recently taken for failing to get a grip on bonuses. Some of it, however, flows straight from the open mind of a genuine intellectual. He has always mixed ideas with finance and politics, his 2001 book Just Capital being hailed as "a gem of liberal political economy worthy of Meade, Keynes and Hobhouse" by no lesser figure than Peter Mandelson. It remains to be seen whether the first secretary will also embrace Lord Turner's latest conclusion that "the idea that more complete markets ... are definitionally good" – an idea New Labour long indulged – has "collapsed" in the face of the credit crunch. Having learnt the hard way that many of those ingenious derivative contracts that were supposed to be about transferring risk were instead concerned with concealing it, the FSA's boss concedes that regulators are in a "worrying space". Their old models of finance have been exposed as bust, and amid all the fire-fighting they have not had any time to sketch a new cognitive map. Their only guide in uncharted territory is gut judgment and the principle of precaution.

With Lehman and other great towers of high finance fallen, the clear and present danger – already visible in recovering bonuses – is that the remaining investment banks, newly unencumbered by competition, could soon be in a position to leech on the wider economy even more freely than they have in the past. European initiatives to restore some control have thus far foundered on the financiers' credible threat to respond by heading off shore. In the US, meanwhile, a president who was more than willing to turn populist heat on the money-lenders during his campaign is too embroiled in trying to force healthcare through to have the stomach for another big row at the moment. But as the G20 in London established, when world's leaders get together in one room, the domain of the possible can extend. The same group is due to reassemble in Pittsburgh next month, and there will be just as much to discuss. For although it can feel as if the moment for reform has already passed, the shake-up of ideas is only just getting going, as Lord Turner has admirably shown.